Allothermal Gasification of Peat and Lignite by a Focused Light Flow
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applied sciences Article Allothermal Gasification of Peat and Lignite by a Focused Light Flow Alexandr S. Zaitsev 1, Roman I. Taburchinov 1, Irina P. Ozerova 1, Amaro O. Pereira, Jr. 2 and Roman I. Egorov 1,* 1 Tomsk Polytechnic University, Tomsk 634050, Russian 2 Institute of Graduate Studies in Engineering at Federal University of Rio de Janeiro, Rio de Janeiro 21941-901, Brazil * Correspondence: [email protected]; Tel.: +7-3822-701-777 (ext. 3485) Received: 21 March 2020; Accepted: 6 April 2020; Published: 11 April 2020 Abstract: Gasification of peat and lignite under a focused light flow was observed in a wide range of fuel moisture (up to 65 wt.%). The initial water content in the fuels under study had a different influence on the chemical composition of the synthesis gas (syngas). At the same time, the effect of light intensity was more predictable: in general, the production of gases grew with it. It was shown that the gasification of peat accelerated greatly when light intensity exceeded 100 W/cm2. Moreover, the conversion of peat and lignite required an order of magnitude lower intensity of the light flow than was necessary for the conversion of bituminous coal processing waste. The dynamics of the sample weight changes demonstrated that contrary to bituminous coals, the process was not purely allothermal for both peat and lignite. However, the fuel smoldering was not self-sustainable and stopped shortly after the pumping light was turned off. Keywords: peat; lignite; allothermal gasification; focused light; synthesis gas 1. Introduction Over recent years, human civilization has gone through a chain of energetic crises. Some of them were caused by the conflicts of interests between the world’s centers of power, when the availability of oil and gas decreased significantly for many countries. However, there were some objective reasons for that, as well: an increase in the global population and the depletion of numerous readily available deposits of fossil fuels that were in use for a long time [1]. The deficit of high-quality industrial fuels moves people to use widespread and low-demand fossil fuels (oil shale, peat, lignite) and other raw materials earlier classified as waste [2–4]. Using different types of domestic waste and biomass as fuel represents yet another way of replacing traditional fuels [5,6]. However, using such quasi-renewable fuels through direct burning calls for a complicated modification of heaters and involves additional processing stages (like esterification) of the fuel. A permanent increase in the atmospheric pollution causes climate changes. Therefore, the new fuels and corresponding techniques of their application have to be environmentally friendly. However, replacing coals, oil, and natural gas by low-quality fuels on a permanent basis is quite expensive and only makes sense in a few industrial sites. A much more attractive option is to preprocess such fuels to a state when a sub-product is suitable for burners with the current design. This is implemented by the thermal conversion of a fuel (pyrolysis and/or gasification) to a combustible gas [5–7]. It typically produces synthesis gas (syngas)—a mixture of combustible and non-combustible gases (CO, CO2,H2, CH4, etc.) with the water steam. Syngas can serve as an inexpensive replacement of natural gas for the power industry or as a good crude material for the chemical industry [8–10]. Therefore, it is a relevant task currently to develop Appl. Sci. 2020, 10, 2640; doi:10.3390/app10082640 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 2640 2 of 11 efficient and inexpensive methods of converting fossil fuels into synthesis gas. The most well-known gasification techniques suggest the conversion of fuel through its partial combustion [11–13]. This method is good, but it requires special efforts to keep the efficiency at a high enough level when the fuel has low reactivity and high ash content [14,15]. In this case, the production of heat has low intensity, and an essential part of it is spent on heating the concomitant passive matter. The second argument for the development of a new gasification method is that a certain heat flow is required for the optimal conversion efficiency. Combustion gives us very few possibilities for heat flow manipulation (the maximum heating intensity is limited by the fuel reactivity, and even reducing the temperatures is not so easy for autothermal processes). Applying focused sunlight as a heat source of fuel gasification promises a relatively inexpensive industrial conversion technique. Taking into account that the light absorption coefficients of fossil fuels are much higher than those of high-quality photovoltaic devices, this approach looks quite promising. There are already some known attempts to use solar light for fuel processing [16–18]. However, these approaches are quite different from the ones proposed in this paper in terms of the types of fuels used, their moisture content, and the fuel particle sizes. An important factor of solid fuel gasification is the reaction of carbon with water [11,12]. On the one hand, it rapidly produces CO and H2, and on the other hand, it suppresses the direct oxidation process, thus preventing the fuel ignition. Consequently, most of the gasification approaches deal with water-fuel compositions [13,19]. The existing allothermal (allothermal processes here are thermochemical processes where the heat needed for the transformation of precursors comes from the external heat sources) approaches are suitable for relatively pure carbon. They are mostly used in the gasification of high-grade bituminous coals (anthracite) and coal coke inside molten metal [20] or within another heat recuperation circuit [21]. Such approaches are unsuitable for peat and lignite due the large amount of passive mineral residue, which pollutes the heat transfer agent of the main heat-producing cycle. Meanwhile, peat is the most calorific type of natural biomass, which is widespread in the world. Peat and lignite are a very attractive class of fuels (or resources for the chemical industry) whose potential can be fully discovered just with an efficient and inexpensive utilization technique. On the contrary, the said technique of the light-induced gasification of coal-derived fuel compositions [22–24] does not have such downsides. It allows a flexible adjustment of the heat flow, as well as ignores the presence of non-combustible admixtures in the fuel. Extending the previously developed approach to fossil fuels of low-level metamorphism is very interesting both scientifically and practically. The main task of this work is to determine the light intensity levels required for the efficient conversion of peat and lignite into syngas, as well as to estimate the effect of fuel moisture content on the chemical composition of gas products. 2. Materials and Methods A fuel layer can be heated by a light flow in different ways. Using solar light has an obvious economic benefit. However, the main parameters of gasification can be determined using artificial incoherent light sources (different lamps, LEDs, etc.) whose spectrum is quite close to certain solar spectrum segments. The absorption of the fuel is not uniform along the visible light spectrum. This fact is easily proven by the color of the fuel powder (both peat and lignite have perceptible reflection in the orange-red region). However, the wet powder of both peat and lignite becomes closer to black, which means higher unification of the absorption spectrum. The samples of both peat (from the Arkadievskoe peat deposit in the Tomsk region of Russia) and lignite (Class B2 by the Russian standards) were dried at 45◦ and milled into a powder with a particle size under 120 µm. The powder was stored in a sealed container up to the final preparation of the sample before each experiment. This allowed high uniformity of the bulk density and heat conductivity of the composition with different water content. A higher concentration of water increases Appl. Sci. 2020, 10, 2640 3 of 11 the threshold value of light intensity above which the fuel ignites, turning gasification into trivial combustion. At the same time, the water content determines the chemical composition of syngas due to changes in the operating temperature and the contribution of the steam-carbon reactions. The total moisture content of samples was varied in the range from 10 to 60 wt.% for peat and from 15 to 65 wt.% for lignite with 10 wt.% steps. The natural moisture content of peat and lignite was 9.9 wt.% and 15 wt.%, respectively. The ash content of peat and lignite was about 22 wt.% and 13.5 wt.%, respectively. Peat contained more than 70 wt.% of bound volatiles, whereas lignite had more than 27 wt.%. Due to such differences in the physical and chemical composition of the fuels, the amount of combustibles in them was different. The sample was placed inside a cylindrical chamber with a window on the top side. The volume of the chamber was about 3.5 cm3, and most of it was free. Therefore, the chamber had some spare air, which was actually enough to gasify a few milligrams of fuel. The chamber was high enough to prevent the window from being polluted by adhering gasification products. The gasification products were permanently pumped out from the chamber by a gas-analyzer pump whose branch pipe was placed below the middle of the chamber height, thus detecting most of the volatile gasification products. The process was done at the normal atmospheric pressure. We used a halogen lamp (total power 490 W and light flow power 12.5 W) with a collimator and focusing optics to heat the fuel. The scheme of the experimental setup and the irradiation spectrum of the lamp are shown in Figure1. The light was focused on a spot with an average diameter (FWHM) of about 2.5 mm.